Field of the Invention
The present invention concerns a method for optimizing a magnetic resonance control sequence, as well as a computer, a magnetic resonance apparatus and a storage medium configured to implement such a method.
Description of the Prior Art
In a magnetic resonance apparatus, the examination object, for example, a patient, is exposed to a relatively strong magnetic field of, for example, 1.5 or 3 Tesla, with the use of a basic field magnet. In addition, a magnetic field gradient is applied by gradient coils. A radio frequency antenna unit, using suitable antenna devices, emits radio frequency excitation signals (RF signals), that cause nuclear spins of particular atoms to be excited into resonance by this high frequency field and thus to be tilted through a defined flip angle relative to the magnetic field lines of the basic magnetic field. During relaxation of the excited nuclear spin, radio frequency signals known as magnetic resonance signals are emitted and are received by suitable receiving antennae and then further processed. From the raw data thereby acquired, the desired image data can be reconstructed.
For a particular measurement, therefore, a specific magnetic resonance control sequence, also known as a pulse sequence, must be transmitted by the gradient coils and the RF antennae. Such a sequence is composed of a series of radio frequency pulses, in particular excitation pulses and refocusing pulses, as well as gradient pulses that are emitted suitably coordinated therewith. The gradient pulses generate dynamic magnetic field gradients in different spatial directions and are used for spatially encoding the magnetic resonance signals. Temporally adapted thereto, readout windows must be set that specify the time frames within which the induced magnetic resonance signals are acquired. Of particular importance for the imaging is the timing within the sequence, that is, the temporal intervals at which pulses follow one another. A large number of the control parameters are typically defined in a so-called scan protocol which is created in advance and can be called upon for a particular measurement, for example, from a memory and, if necessary, can be adjusted by the user on site, who can specify additional control parameters such as a particular slice increment of a batch of slices to be scanned, a slice thickness, etc. Then, based on all these control parameters, a magnetic resonance control sequence is calculated in the control computer of the magnetic resonance apparatus.
The gradient pulses and the corresponding magnetic field gradients are defined by the gradient amplitude G, the pulse duration, and the edge steepness or the first derivative dG/dt of the pulse form, which is the shape over time of the gradient amplitude, usually designated the “slew rate”. A further important gradient variable is the magnetic moment also called the gradient moment, which is defined as the integral of the gradient amplitude over time.
During a magnetic resonance control sequence, the gradient coils, by which the gradient pulses are emitted, are switched frequently and rapidly. Since the time pre-selections within a magnetic resonance control sequence are mostly very strict, and the overall duration of a magnetic resonance control sequence, which determines the overall duration of an examination, must be kept as small as possible, gradient amplitudes of approximately 40 mT/m and slew rates of up to 200 mT/m/ms must sometimes be achieved. Such a high edge steepness contributes to the known noise manifestations during switching of the gradients. Eddy currents in other components of the magnetic resonance device, in particular the radio-frequency shield, are a reason for this noise nuisance. In addition, steep flanks of the gradients lead to a higher energy usage and also place greater demands on the gradient coils and other hardware. The rapidly changing magnetic field gradients lead to distortions and oscillations in the gradient coils and to the transference of these energies to the scanner housing. Due to heating of the coils and the other components, a high degree of helium boil-off can also occur, if the basic field magnet has helium-cooled superconducting coils.
To reduce the noise nuisance, various solutions have been proposed regarding the construction of the hardware, such as encapsulation or vacuum-sealing the gradient coils, and optimization processes for magnetic resonance control sequences.